This application claims priority to Great Britain Application No. 0417668.1, filed Aug. 6, 2004. The above-listed application is hereby incorporated in its entirety herein by reference for all purposes.
The present invention relates to a control system and more particularly to a joystick type control system, and particularly to such systems utilizing magnetic positional sensing used in safety critical human/machine control interfaces.
Various uses for joystick control systems, such as the present invention, include wheelchairs, forklift trucks or other man-carrying vehicles, and control of machines such as cranes, robots or other industrial equipment where a dangerous situation could exist in the event of a control system failure. In such a system, dual joystick position sensor channels may be used, and the outputs compared to one another continuously. This ensures that if there is a problem with one of the sensor channels, the error is picked up due to a mismatch in the outputs at the 2 channels. If a discrepant output (differential beyond a predetermined threshold) occurs, the control system rapidly and safely disables the system.
The force with which a user operates the controller and, to a lesser extent, manufacturing tolerances, can result in the joystick shaft shifting in position translationally in the three orthogonal directions (x,y,z). Due to such tolerances and the fact that the primary and back up sensor in each fail-safe pair cannot occupy exactly the same position in space, the outputs from the sensors in the pair will differ slightly and allowance must be made for this when setting the tolerance threshold. The sensors are typically programmable, allowing each pair to be calibrated to provide a zero difference in output from each sensor of the pair, under normal operating conditions. However, if the threshold is too small then the monitoring system may indicate a malfunction, creating false errors referred to as nuisance trips in the art.
Alternatively, the sensors in each pair could be arranged to provide outputs having opposite sense. In such an implementation, the output of one sensor of the pair could be arranged to provide a positive output, and the other sensor of the pair could be arranged to provide a negative output. However, in both arrangements, the sum of the outputs of the sensors in a given pair, or their mean, is required to be a constant to within the tolerance threshold.
For joystick systems of the magnetic sensing type, it is necessary to measure the angular position of the joystick shaft (and therefore the magnet) without introducing errors due to the linear motion of the magnet in the three orthogonal directions. There is thus a need for an improved control system.